How long does it take to build an ADU in Arizona?

A new detached ADU (accessory dwelling unit, often called a casita in Arizona) usually takes 12 to 18 months from first sketch to move-in, and the time splits into three parts: design, approvals and construction. Construction alone typically runs 6 to 12 months for a site-built unit, by one county ADU program's published figures. Plan review and permitting take 2 to 4 months, depending on jurisdiction. Published city targets, such as Scottsdale's 7 business days for its administrative check and 15 for substantive review, cover one review pass, not the whole approval. A garage or interior conversion is often faster than a new detached unit, because it reuses a slab, walls and roof that are already there. What most often stretches an ADU schedule is not the build. It is incomplete plans, rounds of corrections, and utility work on rural lots.

Below is how the months break down, what each phase involves in metro Phoenix, and what speeds a project up or slows it down.

The timeline phase by phase

Every ADU moves through the same phases, whatever its size. These are planning ranges, not promises, since each lot and each city is different.

• Lot check and design. Confirm the zoning, the size cap, setbacks and lot coverage, then draw the unit and its construction documents. A simple single-story casita designs faster than a unit with complex structure or a hillside site.
• City or county approval. Plan review against the building code, plus any extra step your jurisdiction adds. In Scottsdale, a pre-application, a pre-application meeting and an administrative approval come before the plans even go to permit review.
• Site work and utilities. Grading, the pad, trenching for water, sewer or septic, and power. Short runs behind an existing home go quickly. Long runs on acreage take longer.
• Construction. Foundation, framing, roof, the mechanical, plumbing and electrical rough-ins, insulation, drywall, finishes and the final inspection.

Alameda County's ADU program in California, one of the few public programs that publishes stage-by-stage figures, lists site preparation at 1 to 2 months, the foundation at about 1 month, walls, roof and doors at 1 to 2 months, plumbing and electrical at 1 to 2 months, insulation and drywall at half a month to a month, and fixtures and finishes at 1 to 2 months. Arizona's dry climate does not shorten that order of work, and our summer heat and monsoon storms can add days to concrete and roofing.

How long ADU permits take in Arizona

Allow 2 to 4 months, depending on jurisdiction, for plan review and permitting. A complete plan set keeps you at the short end. An ADU is a permitted use in cities over 75,000 people and in every county since January 1, 2026, so there is no public hearing to schedule. The city statute defines a permitted use as one approved "without requiring a public hearing, variance, conditional use permit, special permit or special exception." What remains is plan review, and its correction rounds.

The review clock depends on your city or county:

• Scottsdale publishes its targets: 7 business days for administrative review and 15 for substantive review on a full residential plan set. Those are targets for one review pass, before any corrections, so they are not the whole permitting time. Add the pre-application and administrative approval that Scottsdale requires for an ADU.
• Phoenix is building an ADU Standard Plan Library of pre-approved detached plans, free to city residents and built as drawn. It is meant to cut design time and cost, though a site plan is still required. When we last read the city's page, the first plans were still in review.
• Unincorporated Maricopa County, which covers the Rio Verde Foothills, takes permits online through its Permit Center. A rural lot may also need its septic system sized for the ADU before construction, which adds its own review.

Each round of corrections adds time, so a clean, complete plan set is the fastest permit. Our pages on how to get an ADU permit in Arizona and how long a building permit takes go step by step.

Why a small unit does not build in a few weeks

An ADU is small, but it is a complete home. It has a kitchen, a bathroom, its own heating and cooling, and its own utility connections, and each of those gets its own rough-in and inspection. The trades still arrive in order: the framer cannot start before the slab cures, and the drywall cannot go up before the rough plumbing, electrical and insulation pass inspection. That sequence is why the construction phase is measured in months, not weeks, even for a unit of a few hundred square feet.

For scale, a whole single-family home in the U.S. took about 7.6 months to build in 2024, after the permit, by the National Association of Home Builders' reading of Census data. An ADU has far less to frame and finish, so it usually takes less, but it keeps every step and every inspection. For a full home's timeline, see how long it takes to build a custom home in Arizona.

What speeds up or slows down an ADU

A few choices move the schedule more than anything else:

• Conversion or detached. A garage or interior conversion skips the new foundation, framing and roof. In Phoenix it does need new off-street parking to replace the garage spaces.
• Complete plans. A plan set with every sheet and calculation clears review on the first pass. Missing items send it back.
• Utilities. A unit next to an existing home with sewer and power close by is quick. A unit on a rural lot that needs septic work, a longer power run or a separate meter takes longer. Scottsdale requires an ADU's utilities to be metered separately from the main home.
• Lot problems found late. An unpermitted shed, a utility easement in the way, or an HOA review can stall a project before it starts. Phoenix asks owners to legalize unpermitted buildings before the ADU permit, and the city statute bars building on a public utility easement without the utility's written consent.
• Building with the main home. When the casita is drawn and permitted with a new custom home, it moves through design, permitting and construction with the house instead of on a schedule of its own.
• Finish choices. Special-order windows, tile or cabinets with long lead times can hold up the last month.

Plan the budget on the same timeline

The schedule and the budget move together. Design and permit fees come due before any work starts, and a construction loan or a draw schedule pays the build in stages as each phase passes inspection. Our page on what an ADU costs to build in Arizona breaks down where the money goes, and our financing page shows how a build can be paid for over the months it takes.

Review times and requirements change with workload and with new rules. Confirm the current numbers with your city, town or county before you set a move-in date.

Building an ADU with Jematell Homes

We build casitas, guest houses and ADUs across the Phoenix metro, with a new home or on a lot you already own, and you get a written progress update every week of construction. Licensed under Arizona ROC# 339367. Tell us about your lot through our contact page and we will walk you through the schedule for it.

Sources

• Alameda County ADU program: Construct (construction stages and timeline)
• City of Scottsdale: Plan Review Services (review time targets)
• City of Scottsdale: Accessory Dwelling Units (application process)
• City of Phoenix: Accessory Dwelling Units and the ADU Standard Plan Library
• A.R.S. 9-461.18 (Accessory dwelling units; municipalities)
• NAHB Eye on Housing: Single-family homes are built faster in 2024